In the recent drive for higher integration and operating speeds in LSI devices, it is desired to miniaturize the pattern rule. Great efforts have been devoted for the development of the micropatterning technology using deep-ultraviolet (deep-UV) or vacuum-ultraviolet (VUV) lithography. The photolithography using KrF excimer laser (wavelength 248 nm) as the light source has already established the main role in the commercial manufacture of semiconductor devices. The lithography using ArF excimer laser (wavelength 193 nm) is under investigation to enable further miniaturization and has reached the stage of prototype manufacture experiments. However, the ArF excimer laser lithography has not matured so that many problems must be overcome before the technology can be applied to an industrial scale of semiconductor manufacture.
The requisite properties for the resist materials complying with the ArF excimer laser lithography include transparency at wavelength 193 nm and dry etch resistance. Resist materials comprising as a base resin poly(meth)acrylic acid derivatives having bulky acid-labile protective groups as typified by 2-ethyl-2-adamantyl and 2-methyl-2-adamantyl groups were proposed as having both the properties (JP-A 9-73173 and JP-A 9-90637). Since then, a variety of materials have been proposed. Most of them commonly use resins having a highly transparent backbone and a carboxylic acid moiety protected with a bulky tertiary alkyl group.
While the prior art resist materials suffer from several problems, the most serious problem is the unevenness of a fine line size generally known as “line edge roughness.” There is a strong desire to solve the problem since such unevenness largely affects the performance of semiconductor devices manufactured. If it suffices to provide the pattern with a smooth finish, the object may be attained to some extent by setting the molecular weight of a resin at a relatively low level or by selecting such a photoacid generator that the acid resulting therefrom is more mobile. These approaches, however, extremely exacerbate such properties as exposure dose dependency, pattern density dependency, and mask fidelity, and do not entail a reduction of line edge roughness because fine fluctuations of the mask are enlarged so that the line size itself rather becomes uneven. While a further reduction of the pattern rule is invariably required, there is a demand for a resist composition which exerts excellent performance in terms of sensitivity, substrate adhesion and etch resistance and is essentially improved in line edge roughness without the sacrifice of resolution.
As the pattern layout becomes finer, the fluctuation of pattern line width, known as “line edge roughness” (LER), becomes significant. In the processing of gate electrode zones in the LSI circuit manufacturing process, for example, low LER can give rise to such problems as current leakage, resulting in a transistor with degraded electrical properties. It is believed that the LER is affected by various factors. The main factor is the poor affinity of a base resin to a developer, that is, low solubility of a base resin in a developer. Since carboxylic acid protective groups commonly used in the art are bulky tertiary alkyl groups and thus highly hydrophobic, most of them are less soluble. Where a high resolution is required as in the formation of microscopic channels, a noticeable LER can lead to an uneven size. One of known approaches for reducing LER is by increasing the amount of photoacid generator added, as described in Journal of Photopolymer Science and Technology, vol. 19, No. 3, 2006, 313-318 and 327-334. This approach, however, exerts a less than satisfactory effect, sometimes at the substantial sacrifice of exposure dose dependency, mask fidelity and/or pattern rectangularity.
Another approach toward LER improvement is the use of a resin containing a fluoroalcohol partial structure, as described in Journal of Photopolymer Science and Technology, vol. 18, No. 3, 2005, 381-387. The introduction of fluoroalcohol improves the solubility of a base resin in an alkaline developer, but entails the negative effect of facilitating acid diffusion during heat treatment following exposure (i.e., post-exposure baking, PEB). This invites degradation of resolution, failing to gain a satisfactory process window (focal depth, exposure margin, etc.).